I'm currently scanning a lot of old slides. The scanner I use has this annoying habit of adding "My beautiul picture" in a description field in the EXIF format.

I'd like to use an EXIF editor to get rid of that stupid message on each file, and add actually useful data, like the date I actually shot those pictures. Ideally, this should be able to work in batch mode (I have hundreds of those files).

I use jhead for simple things like this. jhead -dc file.jpg will delete the comment field, or you can use jhead -cl "new comment" file.jpg to replace it. It also has a number of options for setting time and date, including taking the new EXIF date from the file's timestamp, or else setting it from the command line.

It's open-source software and comes with pre-built versions for Linux, OS X, and Windows. It's pretty simple, but is a command-line tool, so if you'd prefer a GUI solution you should look elsewhere. As a CLI program, though, it's nice for scripting so you can automatically fix all of your images every time you import.

If you have more complex needs, ExifTool is much more powerful — and complicated.